The present invention relates generally to the formation of pockets in surfaces. More specifically, it is directed to the formation of discrete oil retention pockets in the walls of engine cylinders.
The diameter and surface finish of engine cylinder walls are important to engine operation, as such characteristics may affect friction between the cylinder and piston rings, ring wear, and blow-by. The process of manufacturing engine cylinders typically includes rough and finish boring operations, followed by rough and finish honing operations, followed by a brushing operation to remove surface protrusions. Conventionally, the honing operations, particularly the finish honing operations, are used to control the final cylinder diameter and surface finish. It is often desirable that, by the end of these steps, reservoirs will be in the cylinder wall to retain sufficient oil for piston-wall lubrication.
Cylinder honing is conventionally done by a rotary tool, which is axially reciprocated within the bore, producing a crosshatch pattern. The feed rate (feed per revolution) may affect the angle of the cross-hatching. This process may produce shear stress in the cylinder wall by tearing or cutting the metal, resulting in metal being pushed over the graphite particles in the cast iron. Plateau honing may remove these high points on the cylinder wall surface. When such plateau honing is done, there may be no “break-in” period during which the piston rings remove the high points during initial engine operation.
Engine blow-by may occur when the air/fuel mixture leaks past the piston rings, into the crankcase, possibly blowing the mixture into the air cleaner. Oil blow-by may occur when oil enters the combustion chamber past the piston rings, oftentimes ending up in the burnt exhaust. Oil blow-by can be a major emission problem. The conventional cylinder honing process may produce angled, relatively long and continuous grooves that span the thickness of the piston rings. This may allow oil to migrate or be pumped or pushed through the grooves past the rings into the combustion chamber, resulting in blow-by. The surface pattern produced by conventional honing processes may be fairly uniform throughout the axis and circumference of the cylinder. The continuity of the grooves produced by this process may increase the potential for blow-by, as oil or an air/fuel mixture has a continuous path from generally one end of a cylinder to the other.
To eliminate the presence of such continuous paths along cylinder walls, and thereby reduce the likelihood of blow-by, lasers have been used to form individual pockets in cylinder walls for the retention of oil. The microstructure produced by such lasers typically comprises pockets (either spiral or cup structures) that are created in the cylinder wall by a laser beam. Typically, a series of dots or dashes 25 to 60 microns deep and 40 microns wide are burned into the top third of the cylinder by the laser after the bore has been semi-finished. After laser machining, the cylinders are usually finish-honed to remove the buildup of any molten mass on each side of the groove around the pits and to finish the bore. The typical automotive cylinder production sequence using laser structuring is thus comprised of rough honing, semi-finish honing, laser structuring, and finish honing. The heat generated by the laser, however, may produce unwanted weakening and/or hardening of the area surrounding the pockets. This hardening may be destructive to piston rings. Other drawbacks of exposing metal to heat, such as that generated by a laser, are known in the art.